1. Introduction
Aquatic systems enable hydrological cycling, climate regulation, and habitat provision for aquatic organisms. Heavy metal pollution in the aquatic environment has attracted global attention because of the environmental toxicity, persistence, and bioaccumulation of heavy metals, which can pose adverse effects on living beings and the entire ecosystem [
1,
2]. Sediment is considered as the largest pool of heavy metals in the aquatic environment [
3,
4]. About 99% of the heavy metals load in aquatic systems has been found to ultimately precipitate onto the sediment [
5]. Heavy metal concentrations in the sediment are usually four or five times higher than that found in the overlying water [
6]. Thus, sediment quality can reflect the heavy metal pollution status of the whole ecosystem. Consequently, it is of great importance to measure the amount of heavy metals in the sediment to provide information on the heavy metal contamination of the entire aquatic ecosystem.
Heavy metals enter aquatic ecosystem sediment via natural processes, including atmosphere deposition, rock weathering and erosion, and hydrodynamic alteration, as well as via anthropogenic activities, such as industrial wastewater discharge and agricultural fertilizer leaching [
7]. With the rapid development of industrialization, urbanization, and agriculture, anthropogenic activities have become the main source of heavy metal pollution in the sediment of many rivers around the world. When the environmental factors of pH, oxidation-reduction potential (Eh), and organic matter [
7,
8,
9] change, the heavy metal in sediment may release into the overlying water. In addition, the flushing operation caused by flow may not only cause the resuspension of sediment, but also affect the spatial distribution of heavy metals in the sediment [
10]. As well as the artificial replenishing activities of sediment that may result in the dispersion of pollutants (include heavy metals) in the sediment [
11,
12]. Accordingly, it is required to have a comprehensive assessment of the sediment quality, considering environmental changes.
Various studies have focused on the assessment of the extent of metal pollution using numerous analytical techniques based on heavy metal concentration and distribution patterns [
13,
14,
15]. Ke et al. [
16] assessed the ecological risk of the heavy metals Cd, As, Cu, Ni, Pb, Cr, and Zn, with sediment quality guidelines (SQGs), geo-accumulation index (
), potential ecological risk index (RI), and risk assessment code (RAC). Zahra et al. [
9] determined the metal accumulation, distribution, and pollution status using the enrichment factor (EF),
, and metal pollution index (MPI) of the Rawal Lake tributary. EF and
take into consideration the enrichment and pollution status of a single element; RI, SQG, and MPI take into consideration the combined effects of heavy metals, while RAC is mainly used to determine the speciation effect of heavy metals. Therefore, it is essential to determine the values of several combined indices for a comprehensive understanding of heavy metal pollution in the sediment.
Located south of the middle reaches of the Yangtze River, Dongting Lake is the second largest freshwater lake in China and has the Xiangjiang River (XR), Zishui River (ZR), Yuanjiang River (YR), and Lishui River (LR) as its main feeding tributaries [
17]. The lake serves more than 600 million inhabitants and plays an important role in providing food and habitats for living beings, maintaining biodiversity, and controlling flooding [
18]. Frequent mining and agricultural cultivation along the four tributaries have made them the primary source of heavy metals in Dongting Lake [
19]. Presently, numerous studies have been conducted to evaluate the toxicity and ecological risk of heavy metals in the main body of Dongting Lake or part of its tributaries [
17,
20,
21]. Nevertheless, there have been only a limited number of studies performed on a comprehensive comparison between the heavy metals in the sediments of the major tributaries listed above. Tributary rivers are the main source of pollutants for a lake and play an important role in maintaining a healthy lake ecosystem [
9,
22]. Therefore, this study is a comprehensive evaluation of heavy metals in the sediment of the four feeding tributary rivers in order to enable the efficient management of heavy metal pollution in Dongting Lake and its tributaries.
Given the importance of heavy metal pollution in sediment, this study was devoted to the analysis of the contamination of heavy metals (Zn, Cr, Cd, Pb, As, and Cu). The primary objectives of this study are as follows: (1) to investigate the spatial distribution of heavy metals in the surface sediment of the four tributaries by comparing them with various background values and previous studies; (2) to assess the level of contamination and potential ecological risk of heavy metals in sediments of the four tributaries using EF, , and RI; and (3) to explore the possible sources of heavy metals in the sediment of the four tributaries using correlation analysis and principal component analysis (PCA).
4. Discussion
Heavy metals are a special group that may mainly be derived from anthropogenic activities, with some contribution from natural sources [
41]. In the present study, concentrations of Zn, Cu, As, Cd, and Pb were significantly higher in XR than other rivers, while significantly lower in LR than other rivers, especially in the midstream and downstream regions of the two rivers. These results indicate that heavy metals in the sediment of the four rivers had a high degree of spatial heterogeneity. Taking the similarity in climate and background values of these rivers into consideration, anthropogenic activities, such as urbanization, industry, agriculture, aquaculture, and tourism development, may be main factors in the spatial differences in the heavy metal concentrations in the sediment [
42]. Hunan Province is rich in non-ferrous metals, with high levels of mining, mineral processing, and smelting, contributing to the high concentrations distributed in the midstream and downstream region of XR [
18,
43,
44,
45]. In addition, many large cities, such as Xiangtan, Zhuzhou, and Changsha, are also located in the midstream and downstream regions of XR [
46]. This urbanization and mineral utilization might explain the high concentration of heavy metals in the sediment of XR. Moreover, the large drainage area of XR may bring non-point source pollutants [
47]. The construction of a dam in the upstream may enhance the flushing of the river [
48], and this may also contribute to the relatively higher heavy metal concentrations in the middle and lower reaches of XR. While the phosphorous chemical production in the upstream region of YR might contribute to the significantly higher Pb concentrations observed, as compared with other sections.
The criteria for heavy metal values in the sediments have been developed by international and national governments, to assess their pollution level and ensure sediment security. In the present study, the concentrations of Zn, Cd, Pb, As, Zn, Cu, and Cr were higher than the UCC, US EPA, TEL, and background value for Dongting Lake sediment. This finding indicates that heavy metals in the sediment might cause adverse effects on the river systems studied. This was corresponded with previous studies that reported serious levels of heavy metal pollution in the sediment of Dongting Lake and its tributaries [
21,
49,
50,
51,
52]. This was especially true for Cd, which was 60-fold higher than the UCC and US EPA guidelines and 7.6-fold higher than the PEL. It is of note that Hunan Province had the highest ranked level of production of Cd, As, Cr, and Pb in China for many years, as well as the highest relative industrial discharge via wastewater [
17]. Furthermore, about 1.7 million tons of fertilizer and pesticides containing Cd, Cr, Pb, As, and other metals, are used annually in the Dongting Lake drainage region [
53]. Therefore, anthropogenic activities surrounding the studied area might be the main reason for the heavy metal pollution in the sediment.
Heavy metal pollution of the surface sediment has been one of the most serious problems facing the Dongting Lake. Ma et al. [
54] assessed heavy metals in the sediment of the six major freshwater lakes in China, namely Dongting Lake, Poyang Lake, Hongze Lake, Chaohu Lake, Taihu Lake, and Hongsi Lake, and the results indicated that Dongting Lake shows the highest ecological risk. Some studies compared the heavy metal concentration in the sediment of Dongting lake and the river inlets, the results indicated that the heavy metal concentration in the sediment of the inlet of the four rivers is higher than other parts of Dongting Lake [
55]. In this study, the mean concentration of Zn, Pb, Cd, and As were higher than those found in Dongting Lake [
21]. These results suggested that the four rivers may be the main sources of heavy metals in the sediment of Dongting Lake. However, the relatively lower metal concentration at the outlet of Chenglingji indicates that the metals that have flown in from rivers are diluted in the Dongting Lake [
56]. The construction of The Three Gorge Dam decreases the sand contribution of Yangtze River to Dongting Lake, and increases the flushing of the Dongting Lake sediment, this may resuspend the sediment into the overlying water, thus decreasing heavy metal contents in the sediment of Dongting Lake [
48].
In this study, Cd had high EF (>10), I
geo (>2), and
(>150) values compared with other metals, indicating that the Cd pollution levels were moderate to high in river sediments and presents a high ecological risk. This corresponds with previous reports that Cd was the main pollutants of all of the heavy metals, and had the highest ecological risk in the sediment of Dongting Lake area [
21,
50,
52]. The Cd in the sediment of the Dongting Lake drainage is mainly in exchangeable and carbonate form, this may contribute to the high ecological risk [
50,
51,
57]. What is more, the sensitivity of Cd to low pH, Eh, and OM (organic matter), may also partly contribute to its high ecological risk in the sediment of studied rivers [
57]. This needs further validation in future study. The RI values for heavy metals in sediments indicate that heavy metals present a considerable ecological risk in XR (RI >300), while they pose a low ecological risk in the other three rivers (RI <150). This was corresponding with the high ecological risk of heavy metals in the sediment of southern and eastern Dongting Lake, while a low ecological risk of western Dongting Lake [
56]. Furthermore, the significantly higher EF (>25),
(>4), and
(>150) values indicate that Cd contributed the most to the high ecological risk of sediments in XR, especially in the midstream and downstream regions. Considering the severe toxicity caused by Cd to the respiratory system, nervous system, immune system, and DNA [
58], special measures are urgently needed to control both point and non-point pollutants, and to ensure the safety of aquatic ecosystem, especially XR.
Using the correlation and PCA analysis, pairs of heavy metals were correlated with each other, with As and Cd, and Cr and Pb, presenting similar variations, respectively, suggesting a high level of consistency in their sources. Furthermore, Cu and Zn were significantly correlated with As, Cd, and Cr, as well as having a high loading in both the first and second components. This suggested that Zn and Cu originated from a mutual source to Cr, As, and Cd. The mean concentrations of Cd and As were 17.7- and 3.0-fold higher than the background value of the Dongting Lake sediment, respectively, indicating an anthropogenic source for the two metals. This is corroborated by the moderate to high levels of Cd pollution, while the As pollution was absent to moderate, as derived by the assessment of EF and
. According to the Hunan Statistical Year Book (2017), the content of Cd and As in the sewage discharged from Hunan Province accounts for 40% and 10.3% of the total discharge across China, respectively. Studies have illustrated that XR is the area most polluted by heavy metals in China, with the heavy metal pollution mainly derived from industrial manufacturing and refined mineral mining [
21,
59,
60]. Therefore, the significantly higher concentration of Cd and As observed in the sediment of XR, compared with the other three rivers, indicates that As and Cd may be mainly produced by industrial wastewater and mineral mining processes. According to the
, EF, and RI values established, the studied sediment was not polluted by Cr, with Pb pollution absent to moderate. Therefore, Cr and Pb may be partly produced by natural sources, however, the distinct spatial distribution patterns indicate that these two metals may derive from different sources. Cr concentrations showed low spatial variation, which suggests a non-point agricultural origin. According to the Huan statistical Year Book (2017), 118,661 tons of pesticides were used in Hunan Province in 2017, with most of these pesticides containing metals including Cr. Thus, Cr may mainly originate from natural sources and agricultural activities [
21]. Zn and Cu originate from both the sources of Cd/As and Cr. Atmospheric and river inflows transport Pb from industrial wastewater and sewage discharge [
25,
61].
This study comprehensively assessed the pollution status of heavy metals in the sediment of main tributaries of Dongting Lake. This provides a reference for aquatic environmental management of heavy metals in Dongting Lake drainage. However, further study is needed for a deeper understanding of the heavy metals in the sediment of studied rivers. Firstly, it is the bioavailability rather than the total concentration decides the toxicity of heavy metals. Therefore, further studies should be conducted on speciation of heavy metals in the sediments of these tributaries to confirm their toxicity. Secondly, although environmental factors of pH, organic matter, and Eh were mentioned in this study, no data were measured and no further correlation were analyzed between these factors and heavy metal concentrations. More studies should be conducted to make clear the relationship between environmental factors and heavy metal speciation in the sediment of the four rivers. Lastly, this study confirmed that heavy metals in the sediment of the four rivers are important sources of Dongting Lake. Deeper studies should be conducted to configure to what extent these heavy metals from the four rivers contribute to the Dongting Lake sediment.